[TowerTalk] Fwd: Change in Frequency As Antenna Height Rises

[Bob K6UJ]

You guys are way above my head :-)
I had no idea the resonant freq goes up and down as the antenna travels 
on its way up.
I thought it just went up on a steep curve till about 15 feet then 
gradually after that.
I can see why some head scratching as we pass thru the "decrease zone"  
"what did I do now"  hihi

73,
Bob
K6UJ



On 6/1/16 2:11 PM, jimlux wrote:
> On 6/1/16 1:45 PM, Hans Hammarquist via TowerTalk wrote:
>>
>> It seems to me that Steve's, G3TXQ and Dan's, AC6LA diagrams coincide 
>> pretty well. You find the lowest frequency at about 1/4 wave and max 
>> frequency at about 1/2 above ground. What I don't understand is how 
>> the frequency start to decrease at the high increases before it 
>> reaches 1/4 wave above ground. In my simple mind the fx should start 
>> to increase as soon as the antenna "leaves round".
>>
>>
>> To me, and obviously I'm wrong, the capacitance to ground should 
>> affect the frequency with the result that the fx should increase when 
>> the ground capacitance decreases up to the point where speed of light 
>> start to have an effect.
>>
>>
>> Anybody with a "simple" explanation?
>>
>
> It's basically "not simple" but what you're really looking at is the 
> mutual impedance of the dipole with the "image" in the soil. If you 
> look at those graphs of mutual Z for two dipoles with variable 
> spacing, you'll see a lot of similarity.
>
> What you see as the feedpoint impedance is the combination of the 
> impedance of the dipole itself in combination with the mutual 
> impedance of the image antenna.
>
> And the "other antenna" is a very lossy one.
>
> With a fair amount of math:
> Chapter 23 in Orfanidis's book http://www.ece.rutgers.edu/~orfanidi/ewa/
>
> http://www.ece.rutgers.edu/~orfanidi/ewa/ch23.pdf   (page 1057, figure 
> 23.3.2)
>
> There's a simpler plot in the ARRL Antenna book, but it's not online, 
> and I don't have it handy.
>
>
>
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